One known type of backlight, described in U.S. Pat. No. 4,612,295 and shown schematically in side view in FIG. 1(a) and in plan view from above in FIG. 1(b), comprises a transparent light-guide 1. The light-guide 1 may be a flat slab-like light-guide or it may be tapered in that its thickness decreases with distance across the light-guide. One or more light sources 2, for example fluorescent tubes, are positioned along one or more edges of the light-guide (only one fluorescent tube is shown in FIGS. 1(a) and 1(b) for simplicity of explanation), and light from the or each light source 2 enters the light-guide 1, and propagates within the light-guide according to the phenomenon of “total internal reflection”. One of the larger plane surfaces of the light-guide is provided with a scattering structure (not shown) so that the light propagating within the light-guide is scattered out of the light-guide with an intensity that is preferably substantially uniform across the area of the light-guide. The light guide 1 is placed behind a transmissive display device denoted generally at 3 in FIG. 1(a), so that light scattered out the light guide passes through the display device 3.
A backlight of the general type shown in FIG. 1 may alternatively use light-emitting diodes (LEDs) as the light sources in place of fluorescent tubes. U.S. Pat. No. 6,904, 225 describes a particular type of scattering structure for extracting light from the light-guide, which comprises a plurality of scatter dots provided on a surface of the light-guide. The scatter dot arrangement, defined by the particular placing of the dots, produces even out-scattered light from the light-guide by varying the number density of scatter dots provided on the light-guide to compensate for variations over the area of the light-guide in the intensity of light propagating within the waveguide.
In particular an LED emits light over a certain angular range, and typically has dimensions of the order of 1 mm whereas a light-guide of a backlight for a display may typically have dimensions of the order of 10 cm or greater. Thus, as shown in FIG. 2 which is taken from U.S. Pat. No. 6,904,225 and which is a plan view from above of the light-guide, some regions 4 of the light-guide receive relatively low intensity of light from the light source 2 when an LED is used as the light source. These regions 4 of the light-guide that receive relatively low intensity of light from the light source 2 are provided with a higher number density of scatter dots 5, so that the intensity of light emitted from the light-guide remains approximately uniform over the area of the light-guide.
A backlight of the general type shown in FIG. 1 is widely used in rear-illuminated displays, especially for small LCD displays used in mobile devices and small monitors. Their application to large monitors is also possible.
In general, backlights of the type shown in FIGS. 1(a) and 1(b) that use LEDs as light sources employ white light LEDs. As described in more detail below, a white light LED comprises a phosphor that absorbs some of the light emitted by the light-emitting element of the LED and re-emits light at a wavelength different from the original emission wavelength of the light-emitting element, so that the output from the LED contains a component re-emitted by the phosphor in addition to the unabsorbed portion of the output from the light-emitting element. Alternatively, backlights of the type shown in FIGS. 1(a) and 1(b) may use single colour LEDs (for example, red, green and blue LEDs)—however, this is less common owing to the need to provide a light transition section in which light from the red, green and blue LEDs can mix to give white light.
FIG. 3 is a schematic cross-section through a typical display package 6 consisting of a transmissive display device 3 and a backlight of the general type shown in FIG. 1, in which LEDs are used as the light source(s) 2. (Only one light source is shown in FIG. 3.) A reflector 11 is preferably disposed behind the light-guide 1, so that any light that is emitted from the rear surface of the light-guide 1, away from the display device 3, is reflected by the reflector back towards the display device 3. This improves the output efficiency of the display package.
The transmissive display device 3 may, for example, be a liquid crystal display device.
The need to ensure good coupling of light from the LED into the light-guide 1 means that the thickness t of the light-guide 1 cannot be significantly less than the diameter d of the LED. LEDs having a diameter of 0.8 mm or even 0.6 mm are now commercially available and, if such LEDs are used, the light-guide 1 is required to have a thickness t of at least 0.8 mm or 0.6 mm. (In the case of a tapered waveguide, the thickness t of the waveguide must be at least 0.8 mm or 0.6 mm at at least one point.)
There is a move, especially in the mobile devices market, towards thinner display packages. This is mainly to allow more convenient handling, to reduce cost and to allow more functionality to be fitted into the casing; moreover, a thinner device is more attractive to consumers. For an. LCD module (for example), the thickness of the backlight unit is a significant part of the overall thickness. It is therefore desirable to reduce the thickness of the backlight unit, to enable the thickness of the display package to be reduced.
An LED is typically manufactured in the form of an LED package which consists of a light-emitting element typically having dimensions of the order of 300 μm, surrounded by a reflector to collimate light output by the light-emitting element. In the case of a white light LED package, the light-emitting element generally emits light in the blue or violet region of the spectrum, and the light-emitting element is further surrounded by a yellow phosphor which absorbs some of the blue/violet light from the light-emitting element and re-radiates in the yellow region of the spectrum; the yellow light re-radiated by the phosphor and the unabsorbed part of the blue/violet light from the light-emitting element combine to give a white light output. The reflector is typically made from resin, with a typical thickness of 60 μm. In an LED package of this construction, blue/violet light from the light-emitting element is incident on the resin reflector, and the resin reflector degrades as a result. It is therefore undesirable to reduce the thickness of the LED package, as this would shorten the lifetime of the LED package.
In principle, the resin reflector could be replaced by a metallic reflector. However, in order to decrease the thickness of the LED package by replacing the resin reflector with a metallic reflector, it would be necessary for the metal reflector to be thinner than the current resin reflector, ie to have a thickness of less than 60 μm. A metallic reflector with a thickness of less than 60 μm would be structurally weak and liable to damage, so that the lifetime of the LED package would again be shortened; such a thin metallic reflector would also be difficult to manufacture.
Similarly, where the backlight is provided with one or more fluorescent tubes as the light source(s), the fluorescent tube(s) will in practice have a diameter of around 0.6-0.8 mm. Fluorescent tubes with a diameter of down to 0.3 mm are known but, as with very small LEDs, such very thin fluorescent tubes have a short lifetime and a low light output.
As is explained with reference to FIG. 2 above, an LED emits light over a certain angular range, and typically has dimensions that are much less than the dimensions of a light-guide of a backlight. It is therefore known to provide a light-guide with a light transition section, or light mixing section, in which light from an LED can diverge to cover the full width of a light-guide or, if a plurality of LEDs are provided, in which light from an LED can diverge so as to meet light emanating from an adjacent LED. U.S. Pat. No. 6, 951,401 discloses a backlight in which such a light transition section is placed parallel to and spaced from the main body of a light-guide, to make the backlight more compact by placing the light transition section behind the main body of the light-guide. This is illustrated in FIG. 4, which is a cross-section through a backlight of U.S. Pat. No. 6,951,401.
In the backlight of U.S. Pat. No. 6,951,401 shown in FIG. 4, light from a light source 2, in this example an LED, is coupled into a light transition panel 7. The light transition panel 7 is disposed parallel to the main body of the light-guide 1. The length of the light transition panel 7 is chosen such that the desired divergence of light from the LED can occur as the light passes along the light transition panel 7. Moreover, U.S. Pat. No. 6,951,401 uses single colour LEDs, and light from the different LEDs is mixed in the light transition panel 7 to give white light. Light leaving the light transition panel 7 is coupled into the main body of the light guide 1 by a curved reflector 8 that defines a curved waveguide. Scattering structures 9 are provided on the rear surface 1a of the light-guide, for scattering light out of the front surface 1b of the light-guide.
In the backlight of U.S. Pat. No. 6,951,401, the thickness of each of the light transition section 7 and the light-guide 1 is required to be at least as great as the diameter of the LED, to ensure good coupling of light into the light transition section and from the light transition section 7 into the light-guide 1.
U.S. Pat. No. 6,371,623 also addresses the need for a light transition section in which light from an LED can diverge to cover the full width of a light-guide. As shown in FIG. 5(a), U.S. Pat. No. 6,371,623 teaches placing a prism sheet 10 in the path of light from an LED, to increase the divergence of light from the LED and so reduce the required length of the light transition section 7.
U.S. Pat. No. 6,371,623 also teaches that the LED may be arranged to emit light approximately perpendicular to the plane of the light-guide 1, as shown in FIG. 5(b). The prism sheet 10 is placed between the LED and the light-guide 1. This allows the length of the backlight system to be reduced—in FIG. 5(a) the length of the backlight system is B+L, where B is the length of the light transition section 7 and L is the length of the light-emitting part of the light-guide, but in FIG. 5(b) the length of the backlight system in the direction of the light-guide is B2+L, where B2 is the component of the length of the light transition section 7 that extends in the direction of the light-guide. The overall length of the light transition section 7 in FIG. 5(b) is given by B1+B2 and, for a given prism sheet, B1+B2=B, where B is the length of light transition section 7 in FIG. 5(a). Thus, the length of the backlight system in the direction of the light-guide in FIG. 5(b) will be less than the length of the backlight system in FIG. 5(a), since B2+L<B+L.
JP-A-4 322 204 describes an illumination system having a light guide and a light source. Light from the light source is received in a light gathering section, and passes through a light guide joint to the light guide.
JP-A-2003 279971 relates to a display having an LC section and a backlight for illuminating the LC section. The backlight section has a light guide plate and a light source. The light source illuminates part of the back surface of the light guide plate, and light is emitted from the front surface of the light guide plate.